Several systems exist for cleaning screens and cylinders used for distributing inks and varnishes necessary for printing. These cleaning operations are necessary each time it is required to pass from the printing of one subject to another on the same printing machine. Cleaning is also required when printing the same subject in very large numbers, since the cylinders or blankets accumulate excess ink that could produce undesired results in the final printing. Cleaning is also necessary when it is necessary to change the type or range of ink used in the printing machine.
The simplest cleaning method consists of soaking a piece of fabric, or imitation fabric such as a non-woven fabric (generally obtained through a mixture of polyester and cellulose pulp together with a small amount of binders of the two basic products), with an appropriate solvent product for printing inks, and using such soaked fabric to clean the printing machine by hand.
A more sophisticated method requires the use of a non-woven fabric manufactured in rolls having the same width as the printing machine. The non-woven fabric rolls are loaded on an automatic device, which is designed to unroll the aforesaid fabric, to spray the non-woven fabric with a printing ink solvent product, and to place the non-woven fabric soaked with solvent in contact with the ink-soiled machine cylinders, making them rotate. Due to the contact between the non-woven solvent-soaked fabric and the ink-coated cylinders, the ink is transferred from the cylinders onto the non-woven fabric. The non-woven fabric, now dirty with ink, is re-wound on an automatic winding shaft, while the clean non-woven fabric left on the roller is unwound, sprayed with solvent and placed in contact with the printing machine cylinders. Through this process, the non-woven fabric is passed from the roll through the cylinders until the roll is totally unwound, thus obtaining several total cleaning cycles of cylinders and printing blankets.
The cleaning system described above is the most widely used and requires an automatic device with various components including the following: a container filled with solvent; a distribution pump for the solvent; a set of nozzles for homogeneous distribution of the solvent on the non-woven fabric; a system to collect excess solvent; and all the equipment necessary for unwinding the clean non-woven fabric and then rewinding the used non-woven fabric roll soiled with printing ink.
A third automatic cleaning system requires the use of non-woven fabric rolls previously soaked with solvent and packed in sealed plastic bags for easy transport and storage. These rolls are placed in an automatic device composed of a clean non-woven fabric unwinding system and a rewinding system for the soiled non-woven fabric. The device used with this method is much more simple in that the use of pre-moistened non-woven fabric does not require a solvent tank, a distribution pump, spray nozzles, or a system to collect excess or waste solvent.
This third cleaning system is described in U.S. Pat. No. 5,368,157 and U.S. Pat. No. 5,974,976, both to Gasparrini et al. The disclosures of the above-cited patents are incorporated by reference herein.
The methods described in the above-cited patents have a variety of drawbacks arising from the prolonged storage of the solvent soaked rolls in a warehouse. Indeed, non-woven materials that are left in contact with solvents for long periods tend to wear out and lose part of their tensile strength. The longer the fabric remains impregnated with the solvent, the greater the loss of strength. The tensile strength is a very important feature because the non-woven fabric is stretched between the two rollers of the cleaning system so as to be rubbed vigorously against the rotating cylinder that is to be cleaned. Laboratory tests and dynamometrical controls run by the Applicant have demonstrated that the most common types of non-woven fabric lose part of their tensile strength when soaked in solvents, reducing their breaking load (N/mm2) by about 20%. The breaking load continues to be reduced progressively the longer the non-woven fabric remains impregnated with the solvent. This condition consequently leads to fabric rotting.
On the other hand, the solvent product loses part of its chemical characteristics by reacting with the non-woven fabric, and, above all, with the bonding agents used for manufacturing the fabric. Moreover, the air that is inevitably contained within the non-woven fabric tends to oxidize the solvent with the subsequent reduction of its cleaning properties.
U.S. Pat. No. 5,974,976, cited above, aims to resolve some of the problems illustrated above through the introduction of a small variant, namely, the extraction of some of the air from the non-woven fabric. However, since the air is not completely extracted, the solvent oxidization is only partially reduced, and consequently, the damage caused may be permanent.
There exists therefore a need for providing new systems for cleaning printing blankets and cylinders used for the distribution of ink and varnish in printing operations, that do not exhibit the disadvantages of manual cleaning methods. Such disadvantages include low efficiency and high cost, complexity of the systems, the need for continuous maintenance, the risk of interruptions during printing operations, and finally the high cost of the automatic cleaning systems. Such automatic cleaning systems comprise several components, including fabric rolls, preferably non-woven fabric rolls, that are impregnated using spraying nozzles at the moment they are to be used on the printing rollers. Other disadvantages include the problems of rotting, breaking load loss, material deterioration, both with regard to the fabric roll and the impregnating solvent, due to contact between the roll and the solvent for long periods. Another disadvantage is the oxidization of the solvent by the air remaining in the non-woven roll material, since even if air is removed from a sealed container in which the roll is stored, such removal of air is, at best, only partial.